Partners and International Organizations
(English)
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Inst. f. Geogr. Wissenschaften, Freie Uni Berlin (D), Centro Sperimentale Valanghe e Difesa Idrogeologica, Arabba di Livinallongo (I), Cemagref, Div. Protection contre les Erosions, St.-Martin-d'Hères (F), Inst. f. Wasserbau u. Kulturtechnik, Karlsruhe (D), Dip. di Ingegneria Civile ed Ambientale Trento (I),, FGG - Hidrotehnicna Smer, Ljubljana (SI), Dip. Territoriale e Sist Agroforest., Legnaro (I)
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Abstract
(English)
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Within the EROSLOPE II research project on dynamics of sediments and water in alpine catchments, the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) studied several aspects of sediment transport and their relation to flood parameters.
The WSL has a long experience in monitoring runoff and sediment transport in forested mountain catchments in the prealpine region of northern Switzerland. In the Erlenbach stream, the bedload transport has been continuously monitored by a hydrophone measurement system since October 1986. Calibration measurements were performed in the field and in the hydraulic laboratory. The analysis shows that the hydrophone impulses can be correlated with the volume of the coarse bedload material passing over the sensors. The method is found to be quite robust: all calibration measurements define a similar relationship extending over several orders of magnitude. A limitation of this method is the rather large variability of the data; the standard error is of the order of 100 % to 200 %. The error is smaller when the measurements are averaged over longer time periods (e.g. days). However, the natural variability of bedload transport intensities extends often over several orders of magnitude, for the same hydraulic conditions, and is therefore larger than the measuring error using hydrophones.
The results of more than 12 years of bedload transport measurements in the Erlenbach stream reveal that the relationship between bedload transport and runoff shows a large variability. For higher flow intensities the general trend inferred from laboratory experiments is confirmed by the field measurements: the bedload transport rate is essentially a function of the discharge above the threshold discharge for initiation of bedload motion. However the measured values of bedload transport are about one order of magnitude smaller than those predicted by semi-empirical equations based on laboratory experiments. At flow conditions close to the threshold discharge, there is a sharp increase in bedload transport and a large variability in transport rates. The high variability may be attributed to substantial and irregular energy dissipation due to form roughness in natural torrents.
Two topographic surveys were carried out in 1993 and in 1995 covering a 500 m long reach of the Erlenbach streambed. It is concluded that the net erosion out of the 500 m long reach was almost negligible during the considered period. It has to be assumed that the sediment input from upstream and the sediment contribution from the sideslopes were more or less in equilibrium with the sediment output at the downstream end of the reach.
In Switzerland, observations on single particle movement were from from 1994 to 1999 in three mountain streams. All the tracer stones were equipped with magnetic kernels to facilitate regular determination of their position in the torrent channel. Five classes of tracer sizes were used. It is observed that the mean transport distances of each tracer class do not vary much with particle size. There is, however, a trend that in most cases the artificial rod particles have somewhat longer transport distances than the artificial disc particles (which both have the same weight). The mean transport distances of the tracer stones show some correlation with the effective runoff volume. A clearer trend is observed if the mean transport distances are plotted against the highest peak discharge of each observation period.
Bedload transport data from 20 mountain streams are compared with a predictive formula and with some laboratory data. A semi-empirical formula is used which relates bedload transport essentially to the discharge conditions and the channel gradient. Overall, data are included from some twenty mountain streams world-wide. The emphasis is put, however, on streams in the European Alps and on streams with channel slopes larger than 5 %. The data shows, that (i) bedload volumes are roughly linearly proportional to effective runoff volumes, and (ii) by introducing the bedslope factor, the data plot much closer together. These findings are in agreement with the semi-empirical formula used for the comparison. However, despite this general agreement, there is a large variation of the 'constant' in the semi-empirical formula when comparing sediment transport data from different streams - especially from smaller and steeper streams - with each other and with laboratory data. When plotting the 'constant' of the bedload transport formula against an approximate relative flow depth, it appears that a part of the decrease in bedload transport 'efficiency' for the smaller and steeper streams is associated with decreasing values of relative flow depth.
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